Image sensor and digital gain compensation method thereof

ABSTRACT

An image sensor and digital gain compensation thereof. The image sensor includes a variable amplification device for amplifying an inputted analog image signal as a variable first gain value, an analog-to-digital conversion unit for converting the amplified analog image signal into a digital image signal, and a digital gain compensation device for comparing the first gain value with a reference gain value and compensating the digital image signal as a digital second gain value when the first gain value is less than the reference gain value.

The present patent application is a continuation of application Ser. No.11/081,480, filed Mar. 15, 2005 now U.S. Pat. No. 7,522,202.

FIELD OF THE INVENTION

The present invention relates to an image sensor; and more particularly,to an image sensor capable of compensating for a loss of a primary colorsignal at a variable amplification unit and a method for compensatingfor a digital gain thereof.

DESCRIPTION OF RELATED ARTS

An image sensor is a device that photographs an image with use of acharacteristic that semiconductor materials are reactive to light. Thatis, pixels of the image sensor detect brightness and wavelengths ofdifferent lights emitted from individual objects and convert thedetected brightness and wavelengths into an electrical value. It is therole of the image sensor to convert this electrical value into a levelthat can be processed as a signal.

FIG. 1 is a block diagram briefly showing a conventional image sensor.

As shown, the image sensor includes: a pixel array unit 10 including N×Mnumbers of unit pixels, where N and M are natural numbers; a variableamplification unit 11; an automatic exposure control and exterior systeminterface unit 12; an analog-to-digital converter (ADC) 13; and an imagesignal processing unit 14.

The pixel array unit 10 is arranged with N number of pixels in rows andM number of pixels in columns to maximize photo-reactiveness of thepixels, so that the pixel array unit 10 is capable of detectinginformation on images provided from an external source. Thus, the pixelarray unit 10 is an essential part of the image sensor.

The automatic exposure control and exterior system interface unit 12controls overall operation of the image sensor with use of a finitestate machine (FSM) and is in charge of interfacing operation withrespect to an exterior system. Although not illustrated, the automaticexposure control and exterior system interface unit 12 includes a batchtype register and as a result, the automatic exposure control andexterior system interface unit 12 is capable of programming variouspieces of information related to internal operation. Also, the automaticexposure control and exterior system interface unit 12 controls overallchip operation based on the programmed information.

Although not illustrated, those outputted signals from the pixel arrayunit 10 are inputted to the variable amplification unit 11, forinstance, a programmable gain amplifier (PGA), through an analog linebuffer unit, a column decoder, an analog bus and so forth. Herein, thevariable amplification unit 11 is in charge of analog signal processingoperation.

That is, the analog line buffer unit detects voltages of pixels in theselected row and stores the detected data. Also, the analog line bufferunit is configured with a plurality of lines to be used for colorinterpolation at a rear end and image signal processing. Among theanalog data stored in the analog line buffer unit, column data valuesselected through a control by a column decoder are transmitted to thevariable amplification unit 11 through the analog bus.

When the detected voltage levels stored at the analog line buffer unitare low, the variable amplification unit 11 amplifies the detectedvoltage levels, and the analog data passed through the variableamplification unit 11 are subjected to color interpolation and then, areconverted into digital values through the ADC 13.

Although not illustrated, the digitally converted data are stored into apixel line memory unit configured with various lines for performingdesigned functions of the image signal processing unit 14 installed atthe rear end.

The image signal processing unit 14 is enabled with various functionsfor improving functionality of the image sensor based on pixel outputvalues stored into the pixel line memory unit. Examples of the variousfunctions of the image signal processing unit 14 are colorinterpolation, color correction, gamma correction, automatic whitebalance, automatic exposure and the like.

Meanwhile, the image sensor has a drawback in fixed pattern noise causedby an offset voltage generated because of a slight difference inconditions of executing manufacturing processes. To compensate for thefixed pattern noise, the image sensor adopts a correlated doublesampling (CDS) mode in which the image sensor reads a reset voltagesignal and a data voltage signal of the individual pixels of the pixelarray unit 10 and outputs a difference between the read reset voltagesignal and the read data voltage signal.

A brightness value, which is the most sensitive component to human eyes,is obtained by employing a method for adjusting a light collection timeof an image sensor or a method for adjusting the variable amplificationunit and compensating for the adjustment as in an appropriate brightnessvalue. For instance, even though the light collection time is the same,brightness values are different from each other depending on amounts ofinputted light.

Among various methods for adjusting a brightness value, the lightcollection time adjustment method makes it possible to adjust thebrightness component into an appropriate level. However, this lightcollection time adjustment method is disadvantageous in that acomplementary metal oxide semiconductor (CMOS) image sensor uses imagesin a line-scanning type and thus, an integration value for theindividual lines varies when the light collection time is not an integermultiple of an inputted light period. As a result of this varyingintegration value, there is a problem of inducing a flicker.

Meanwhile, in case that the brightness value is controlled by adjustinga gain of the variable amplification unit, banding noise, i.e., theflicker noise, can be minimized; however, noise generated from pixels isamplified. Therefore, information on an average brightness value withrespect to images provided from an external source is compared with atargeted brightness value based on an internal FSM algorithm to selectan adequate brightness adjustment method corresponding to a currentlyapplied environment. If the targeted brightness value is greater thanthe average brightness value when a user environment is changed from adark site to a bright site, the gain value of the variable amplificationunit is set to decrease towards a negative direction for each definedstep.

In more detail, when a large amount of light is continuously inputted tothe image sensor, the light collection time decreases since the averagebrightness is greater than the targeted brightness. As a result, if thelight collection time is not an integer multiple of the inputted light,the gain value of the variable amplification unit gets to decrease.Especially, a gain value of an analog amplification unit acts as adecremental gain to an inputted signal and thus, being less than areference gain value which is an integer multiple of 1 in the variableamplification unit.

When a signal level becomes low as the signal value inputted from thepixel array unit 10 is multiplied with a multiple less than the integer1, an output value of the variable amplification unit is in a range ofan input value of the ADC 13, thereby decreasing a dynamic range of asignal level. At this time, a saturation code value of a normal signallevel decreases because of the integer multiple less than 1 and thus,being disabled to be saturated. Eventually, the inputted image isoutputted in a distorted state.

As described above, when the conventional image sensor is continuouslyexposed to the bright environment, all data for primary color signals atan active automatic exposure device become saturated. Thus, it ispreferable to multiply the gain value of the variable amplificationunit, which becomes high due to this continuous exposure, with themultiple less than 1. However, in cast that such analog variableamplification unit is subjected to the multiplication by a decrementalmultiple, the signal level inputted to the ADC decreases. Hence, it maybe difficult to secure an intended dynamic range of the signal level.Also, this difficulty may further result in a problem in that colorsignals may not be saturated.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide an imagesensor capable of preventing a decrease in a dynamic signal level causedby multiplication of a decremental multiple at a variable amplificationunit and a digital gain compensation method thereof.

In accordance with an aspect of the present invention, there is providedan image sensor, including: a variable amplification unit for amplifyingan inputted analog image signal as a variable first gain value; ananalog-to-digital conversion unit for converting the amplified analogimage signal into a digital image signal; and a digital gaincompensation unit for comparing the first gain value with a referencegain value and compensating the digital image signal as a digital secondgain value when the first gain value is less than the reference gainvalue.

In accordance with another aspect of the present invention, there isprovided a method for compensating for a digital gain in an imagesensor, including the steps of: amplifying an inputted analog imagesignal as a first gain value being variable; converting the amplifiedanalog image signal into a digital image signal; comparing the firstgain value with a reference gain value; and compensating the digitalimage signal as a second gain value being digital when the first gainvalue is less than the reference gain value.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome better understood with respect to the following description ofthe preferred embodiments given in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram briefly showing a conventional image sensor;

FIG. 2 is a block diagram showing an image sensor in accordance with apreferred embodiment of the present invention;

FIG. 3 is a diagram showing changes in operation areas of ananalog-to-digital conversion unit and saturation codes in case that adecremental multiple of a variable amplification unit is approximately0.6 in accordance with the preferred embodiment of the presentinvention; and

FIG. 4 is a flowchart schematically describing a method for compensatingfor a digital gain of an image sensor in accordance with the preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An image sensor and a digital gain compensation method thereof inaccordance with a preferred embodiment of the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 2 is a block diagram showing an image sensor in accordance with apreferred embodiment of the present invention.

As shown, the image sensor includes: a pixel array unit 20; a variableamplification unit 21; an automatic exposure control and exterior systeminterface unit 22; an analog-to-digital conversion (ADC) unit 23; adigital gain compensation unit 25; and an image signal processing unit24.

The pixel array unit 20 includes N number of pixels in rows and M numberof pixels in columns to maximize reactiveness to light. Herein, N and Mare integers. The pixel array unit 20 detects information on images froman external source and outputs the detected information in a signallevel, for instance, an analog signal in the format of red, green andblue (RGB).

The automatic exposure control and exterior system interface unit 22controls overall operation of the image sensor by using a finite statemachine (FSM) algorithm and, is in charge of interfacing operation foran exterior system. Although not illustrated, the automatic exposurecontrol and exterior system interface unit 22 includes a batch register.Thus, the automatic exposure control and exterior system interface unit22 is programmable for various internal operations. Also, the automaticexposure control and exterior system interface unit 22 controlsoperation of overall chips based on the programmed information.

The variable amplification unit 21 serves a role in receiving an analogimage signal and amplifying the received analog image signal as avariable first gain value.

The ADC unit 23 serves a role in converting a variably amplified analogimage signal into a digital signal.

The digital gain compensation unit 25 compares the first gain value witha reference gain value and, if the first gain value is less than thereference gain value, the digital image signal is compensated as asecond gain value.

The image signal processing unit 24 receives the compensated digitalimage signal and adjusting color interpolation, color calibration, gammacalibration, automatic white balance and automatic exposure.

Although not illustrated in FIG. 2, the signals outputted from the pixelarray unit 20 are inputted to the variable amplification unit 21 whichperforms analog signal processing operation through employing an analogline buffer unit, a column decoder and an analog bus. A programmablegain amplifier (PGA) is an exemplary device to which the signals fromthe pixel array unit 20 are inputted.

That is, the analog line buffer unit detects voltages of the pixels inthe selected row and stores the detected voltages. Also, the analog linebuffer unit is configured with a plurality of lines used for colorinterpolation and a signal processing at a rear end. Column dataselected among analog data stored into the analog line buffer unitthrough a control of the column decoder are transmitted to the variableamplification unit 21 through the analog bus.

When those voltages of the pixels are low at the analog line bufferunit, the variable amplification unit 21 amplifies the low voltagelevels. Then, the analog data passed through the variable amplificationunit 21 are subjected to color interpolation and other processes and aresubsequently converted into digital values by the ADC unit 23.

The variable first gain value of the variable amplification unit 21 isadjusted correspondingly based on an average brightness value of theinputted image data.

The individual digital data are compensated as a second gain valuethrough the use of the digital gain compensation unit 25 when the firstgain value is less than the reference gain value. Although notillustrated, this second gain value is stored into a pixel line memoryunit configured with a plurality of lines for performing variousfunctions of the image signal processing unit 24 at the rear end.

As mentioned, the image signal processing unit 24 includes variousfunctions for improving functionality of the image sensor based on thepixel output values stored into the pixel line memory unit. Examples ofthe functions of image signal processing unit 24 are colorinterpolation, color calibration, gamma calibration, automatic whitebalance, automatic exposure and the like.

Meanwhile, in the image sensor, there is generated inevitable fixedpattern noise caused by an offset voltage since reliability of the imagesensor is not perfectly coherent because of a slight difference inmanufacturing conditions. To compensate for the fixed pattern noise, theimage sensor adopts a correlated double sampling (CDS) mode thatsequentially reads a reset signal and a data signal at each pixel of thepixel array unit 20 and outputs a difference between the read restsignal and the read data signal thereafter.

The above preferred embodiment exemplifies that the reference gain valueis approximately 1. However, the reference gain value is in a range fromapproximately 1.5 to approximately 1.75 with consideration of variousfactors during the actual circuit operation. Also, the second gain valueis obtained by dividing the reference gain value by the first gainvalue. The digital gain compensation unit 25 includes a multiplier andthe like. The first gain value ranges from approximately 0.5 toapproximately 2.5, and the image signal is in the RGB format.

FIG. 3 is a diagram showing changes in operation areas of an ADC unitand saturation codes when a decremental multiple of a variableamplification unit is approximately 0.6 in accordance with the presentinvention.

As an average brightness value of image signals outputted from a pixelarray unit 30 is greater than a targeted brightness value, a variableamplification unit 31 carries out the amplification to attenuate a levelof the image signal in a decremental multiple of approximately 0.6. Thatis, the image signal level decreases by about 60% of an original 255code. That is, the image signal level decreases from an original 255code to a 153 code. Therefore, when the attenuation occurs at thevariable amplification unit 31, an automatic gain compensation unit 33compensates for the signal attenuation by multiplying a second gainvalue which is an inverse of a first gain value of the variableamplification unit 31, i.e., approximately 1/0.6, with an image signalconverted into a digital image signal through the use of the ADC unit32. At this time, inversing the first gain value for estimating thesecond gain value takes place when a reference gain value is 1.Therefore, the digital image signal provided from an image signalprocessing unit 34 is adjusted into the original 255 code, which is theoriginal signal level.

To prevent the signal level from decreasing at the variableamplification unit 31 and adjusts an image in a full scale of theoriginal code from approximately 0 to approximately 255, a gain value ofthe variable amplification unit 31 is compared with an anti-banding gainvalue, and according to the comparison result, a percentage of the gainis augmented to restore a decreased range of the signal level.

FIG. 4 is a flowchart schematically describing a digital gaincompensation method of an image sensor in accordance with the presentinvention. Herein, the same reference numerals are used for the sameconfiguration elements described in FIG. 2.

A large amount of light is continuously inputted to an image sensor froman active automatic exposure device, and then, an analog image signal isoutputted through a pixel array unit 21. Once the analog image signal isoutputted, at step S401, the variable amplification unit 21 adjusts asignal level based on a first gain value, i.e., the decremental multipleof approximately 0.6, so as to adjust a data on saturated primary colorsignals. From this adjustment by the variable amplification unit 21, RGBcolor signals of which code decreases by approximately 102 codes areoutputted. On the other hand, if a small amount of light is inputted tothe image sensor from the active automatic exposure device, the firstgain value becomes greater than a reference gain value, in this case,‘1’.

When the data on primary color signals is adjusted in the aforementioneddecremental multiple of approximately 0.6, a maximum code range at anarea for improving images in later steps is approximately 153, therebyexpressing primary color signals within this maximum code range ofapproximately 153. Thus, this adjustment results in outputs of theseverely distorted primary color signals instead of the exactly intendedprimary color signals.

Next, at step S402, the data on primary color signals amplified into thefirst gain value is converted into digital image signal through the ADCunit 23.

At step S403, to make a compensation for a dynamic data range of a 255code, the first gain value of the variable amplification unit 23 iscompared with the reference gain value in the course of applying theabove adjustment with the decremental multiple of approximately 0.6. Atstep S404, when it is determined to apply the decremental multiple, thedigital image signals are compensated as a second gain value. That is,the variably amplified primary color signals are multiplied with amultiple obtained by dividing the reference gain value by the first gainvalue. As a result, pixel data are secured with the original 255 code,thereby outputting image data compensated with the digital gain.

In accordance with the present invention, the digital gain compensationunit enabled with changing a digital gain value by being programmable bya user is employed to compensate for a range of code losses of theprimary color signals caused by a decremental multiple of the variableamplification unit when a large amount of light is continuously inputtedto the image sensor. A multiplier is one example of the digital gaincompensation unit. Thus, through a simple configuration of hardware likethe multiplier, it is possible to secure a dynamic range of signals andmaintain a saturation range consistently. As a result, there is furtherprovided effect on restoration of the accurate image information.

When an image sensor is exposed to various environments during designingof chips for the image sensor for adjustment of an automatic exposure, adynamic range of the signal varies depending on the displacedenvironment. According to the present invention, these varying signallevels are expressed with codes uniformly ranging from approximately 0to approximately 255 and, the dynamic range is secured such that whitecolor is expressed much whiter at a higher code. This effect furthermakes it possible to maintain a saturation range uniformly.

The present application contains subject matter related to the Koreanpatent application No. KR 2004-0031993, filed in the Korean PatentOffice on May 6, 2004, the entire contents of which being incorporatedherein by reference.

While the present invention has been described with respect to certainpreferred embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

What is claimed is:
 1. An image sensor, comprising: a variableamplification means for amplifying an input analog image signal as afirst gain value; an analog-to-digital conversion means for convertingthe amplified analog image signal into a digital image signal; and adigital gain compensation means for comparing the first gain value witha reference gain value and compensating the digital image signal as adigital second gain value responsive to the first gain value being lessthan the reference gain value.
 2. The image sensor of claim 1, whereinthe reference gain value is approximately
 1. 3. The image sensor ofclaim 1, wherein the digital gain compensation means performs amultiplication operation.
 4. The image sensor of claim 1, wherein thefirst gain value ranges from approximately 0.5 to approximately 2.5. 5.The image sensor of claim 1, wherein the image signal is in a format ofred, green and blue (RGB).
 6. The image sensor of claim 1, furtherincluding a pixel array unit for outputting the analog image signal. 7.The image sensor of claim 1, further including an image signalprocessing means for receiving the compensated digital image signal andadjusting color interpolation, color calibration, gamma calibration,automatic white balance and automatic exposure.
 8. A method comprising:amplifying an input analog image signal as a first gain value;converting the amplified analog image signal into a digital imagesignal; comparing the first gain value with a reference gain value; andcompensating the digital image signal as a second gain value in responseto the first gain value being less than the reference gain value.
 9. Themethod of claim 8, wherein the reference gain value is approximately 1.10. The method of claim 8, further comprising outputting the digitalimage signal responsive to the first gain value being greater than thereference gain value.
 11. The method of claim 9, wherein the first gainvalue ranges from approximately 0.5 to approximately 2.5.
 12. An imagesensor, comprising: a variable amplification unit configured to amplifyan input analog image signal as a first gain value; an analog-to-digitalconverter configured to convert the amplified analog image signal into adigital image signal; and a digital gain compensation unit configured tocompare the first gain value with a reference gain value and compensatethe digital image signal as a digital second gain value responsive tothe first gain value being less than the reference gain value.
 13. Theimage sensor of claim 12, wherein the reference gain value isapproximately
 1. 14. The image sensor of claim 12, wherein the secondgain value is obtained by dividing the reference gain value by the firstgain value.
 15. The image sensor of claim 12, wherein the digital gaincompensation unit is configured to perform a multiplication operation.16. The image sensor of claim 12, wherein the first gain value rangesfrom approximately 0.5 to approximately 2.5.
 17. The image sensor ofclaim 12, wherein the image signal is in a format of red, green and blue(RGB).
 18. The image sensor of claim 12, further including a pixel arrayunit configured to output the analog image signal.
 19. The image sensorof claim 12, further including an image signal processor configured toreceive the compensated digital image signal and adjust colorinterpolation, color calibration, gamma calibration, automatic whitebalance and automatic exposure.